451-File transfer failed. File contains records that are longer than the LRECL of the new file.
451 Transfer aborted due to file error. File is deleted.
421 Service not available, remote server has closed connection

Please help/guide to do ftp XML files to mainframe that can range up to 15 GB

What are you doing on the source machine to ensure that no record is longer than 32760 bytes? You ABSOLUTELY must put record terminators (CR/LF on Windows or LF on Unix) at intervals of no more than 32760 bytes to split the source file into records on the mainframe. You may have to have a corresponding process to recombine the records into one when retrieving the data from the mainframe. If you are going to disk, putting record terminators every 27996 or 27997 bytes (for Windows or Unix) would maximize disk space efficiency.

And what you posted doesn't really make sense -- UNIT=TAPE would not require any cylinders of space; if you use cylinders than TAPE is not an option. And 15 GB is about 19,176 cylinders at half-track blocking (the most efficient).

Can you please share the sample code snippet which does the record splitting at the mainframe end and also which uses the cylinders with maximum efficiency as i 'm a novice to mainframe FTP'ing process

Can you please share the sample code snippet which does the record splitting at the mainframe end

I don't think you understand yet -- the data received in z/OS on the mainframe must have records no longer than 32760 bytes; you CANNOT convert it on the mainframe. I don't recall what the limit is for a Unix System Services file, but I suspect 15 GB might be over the limit. I don't know if your source machine is Unix or Windows, but you (or somebody) will have to write a program in the language of your choice to split the file into records BEFORE it is transferred to z/OS.

For a tape, 32760 bytes is most efficient as that's the longest block size you can use (barring special circumstances). For disk, half-track blocking is typically most efficient which is 27998 bytes.